1. Field of the Invention
This invention relates to asphalt compositions and processes for the economical and environmentally sound disposal of solvent deasphalting bottoms.
2. Description of Related Art
Asphalt, also referred to as mineral pitch, is obtained from natural deposits such as native asphalt, or derived as a product from crude oil or other hydrocarbon refinery processes, known as petroleum asphalt. Asphalt is generally a mixture of hydrocarbons known as bitumen and typically takes the form of a sticky, black and highly viscous liquid or semi-solid. Modern petroleum asphalt has the same durable qualities as naturally occurring asphalt, with the added advantage of being refined to a typically uniform condition free from organic and mineral impurities.
Asphalt is a strong, versatile, and weather and chemical-resistant binding material that adapts itself to a variety of uses. Asphalt binds aggregate such as crushed stone and gravel into a mixture commonly known as asphaltic concrete, which is formed into firm, tough surfaces for roadways and airport runways, as is very well known. A majority of petroleum asphalt produced today is used for the surfacing of roadways and runways. Asphalt paving material is a dull black mixture of asphalt and aggregate, including sand and crushed stone. Asphalt paving material is typically transported and dumped while hot onto a road surface, leveled, and compacted. Hot liquid asphalt is also used alone for expansion joints and patches on concrete roads. Airport runways, tennis courts, playgrounds, and floors of certain buildings also employ asphalt. Light forms of petroleum asphalt known as road oils are sprayed on roadways to settle dust and bind gravel.
Another major use of asphalt is in asphalt shingles and roofing paper or felt. The asphalt helps to preserve and waterproof the roofing material. Other applications for asphalt include waterproofing tunnels, bridges, dams and reservoirs; rust-proofing and sound-proofing metal pipes and automotive under-bodies; and soundproofing walls and ceilings.
In a typical refining process, crude oil is initially heated followed by separation into its various fractions through distillation. The lighter and more volatile components, or fractions, are vaporized and drawn off through a series of distillation levels. The fractions are then generally refined into naphtha or gasoline (considered a “light” distillate), kerosene or jet fuel (considered a “medium” distillate), gas oil or diesel oil (considered a “heavy” distillate), and other useful petroleum products. The heavy residue from this atmospheric distillation process is commonly referred to as topped crude, residual oil or atmospheric residue. This topped crude can be used for fuel oil, or further processed into other products including asphalt. Since asphalt is a heavy constituent of crude petroleum, it does not evaporate or boil off during the distillation process and remains as bottoms. Petroleum asphalt can also be produced from other intermediate refining process units such as hydroprocessing, visbreaking, coking, and solvent deasphalting.
Residual oil can be processed in certain applications by vacuum distillation to remove enough boiling fractions to produce “straight run” asphalt. However, if the topped crude contains enough low volatility, high-boiling components which cannot be economically removed through distillation, solvent extraction, also known as solvent deasphalting, may be required to produce asphalt of the desired consistency.
Solvent deasphalting is a process employed in oil refineries to extract valuable components from residual oil. The extracted components can be further processed in refineries where they are cracked and converted into valuable lighter fractions, such as gasoline and diesel. Suitable residual oil feedstocks which can be used in solvent deasphalting processes include, for example, atmospheric tower bottoms, vacuum tower bottoms, crude oil, topped crude oils, coal oil extract, shale oils, and oils recovered from tar sands. Solvent deasphalting processes are well known and described, for instance, in Yan U.S. Pat. No. 3,968,023, Beavon U.S. Pat. No. 4,017,383, and Bushnell et al. U.S. Pat. No. 4,125,458, all of which disclosures are incorporated herein by reference.
In a typical solvent deasphalting process, a light hydrocarbon solvent, usually one or more paraffinic compounds, is admixed with a residual oil feed and is processed to separate the flocculated solid from the oil mixture. Common solvents and their mixtures used in deasphalting include normal and/or iso paraffins with carbon numbers ranging from 1-7, preferably from 3-7, including methane, ethane, propane, butane, iso-butane, pentane, iso-pentane, neo-pentane, hexane, and iso-hexane. Under elevated temperatures and pressures, generally below the critical temperature, in an asphaltene separator, the mixture is separated into two liquid streams, including (1) a substantially asphaltenes-free stream of deasphalted oil, including resins, and (2) a mixture of asphaltenes and solvent including some dissolved deasphalted oil.
The substantially asphaltenes-free mixture of deasphalted oil and solvent is normally passed to a solvent recovery system. The solvent recovery system of an solvent deasphalting unit extracts a fraction of the solvent from the solvent-rich deasphalted oil by boiling off the solvent, commonly using steam or hot oil from heaters. The solvent is recycled and sent back for use in the solvent deasphalting unit.
In some processes the deasphalted oil fraction is also separated in to a resins fraction and a resins-free fraction. “Resins” as used herein means materials that have been separated and obtained from a solvent deasphalting unit. Resins are denser and heavier than deasphalted oil, e.g., maltenes, but lighter than asphaltenes. The resin product usually comprises aromatic hydrocarbons with highly aliphatic-substituted side chains, and can also include metals, such as nickel and vanadium. Generally, the resins are composed of material remaining after removal of asphaltenes and deasphalted oil.
Watkins U.S. Pat. No. 3,775,292 teaches a solvent deasphalting process where feedstock is deasphalted using a solvent, and then the resin is removed using a selective solvent in a solvent extraction unit so as to provide solvent-lean resin concentrate and a de-resined second liquid phase. Neither solvent is recovered, and the resin and the deasphalted oil is further processed in a hydrocracking unit so as to produce lower boiling hydrocarbons.
Crowley U.S. Pat. No. 4,101,415 and Ven Driesen et al. U.S. Pat. No. 4,686,028 teach similar solvent deasphalting processes in which a feedstock is subjected to a solvent extraction step that removes both the asphaltenes and the resin, resulting in an asphaltene-free and resin-free deasphalted oil. The asphaltene/resin mixture removed from the feedstock is then subjected to a second solvent extraction step that separates the resins from the asphaltenes.
Asphalt can also be blended or “cut back” with a volatile substance, resulting in a product that is soft and workable at a lower temperature than pure asphalt. When the cut-back asphalt is used for paving or construction, the volatile cutter solvent evaporates when exposed to air or heat, leaving hardened asphalt behind. The volatility of the cutting solvent classifies the cutback asphalt as slow, medium, or rapid-curing asphalt. For example, heated asphalt that is mixed with residual asphaltic oil from the earlier distillation process is described as slow-curing asphalt. Asphalt including blends of gasoline or naphtha is described as medium-curing asphalt, and blends of kerosene is described as rapid-curing asphalt.
In addition, asphalt can be emulsified in water to produce a liquid that can be easily pumped through pipes, mixed with aggregate, or sprayed through nozzles. The asphalt is ground into globules of about 5 to 10 microns or less, and mixed with water and an emulsifying agent. The emulsifying agent reduces the tendency of the asphalt and water to separate, and can be colloidal clay, soluble or insoluble silicates, soap, or other oils.
Asphalt can also be pulverized to produce a powdered material. The asphalt is crushed and passed through a series of fine mesh sieves to obtain uniform size granules. Powered asphalt can be mixed with road oil and aggregate for pavement construction. The heat and pressure in the road slowly amalgamates the powder with the aggregate and binding oil, and the substance hardens to a consistency similar to regular asphaltic concrete.
If asphalt is to be used for a purpose other than paving, such as roofing, pipe coating, or as an automotive under-coating sealant or water-proofing material, it may be oxidized, typically referred to as “air blown” asphalt. This process produces a material that softens at a higher temperature than paving asphalts. The material can be air blown at the refinery, at an asphalt processing plant, or at a roofing material plant. The asphalt is heated to about 260° C., and air is bubbled through the asphalt for 1 to 4.5 hours. When cooled to ambient temperatures, the asphalt remains in a liquid phase.
Two main types of asphaltic concrete compositions include hot-mix and cold-mix. Cold-mix asphalt generally incorporates emulsified or cut-back asphalts, and is usually used for light to medium traffic secondary roads, in remote locations or for maintenance use.
Hot-mix asphalt is commonly used for heavier traffic, and is a mixture of suitable aggregate coated with asphalt. The term “hot-mix” is derived from the process of heating the aggregate and asphalt before mixing to remove moisture from the aggregate and to obtain sufficient fluidity of the asphalt for proper mixing and work-ability. Asphalt and aggregate are combined in a mixing facility where they are heated, proportioned, and mixed to produce the desired paving mixture. Hot-mix facilities may be permanently located (also called “stationary” facilities), or it may be portable and situated on site, and can be classified as either a batch facility or a drum-mix facility. Batch-type hot-mixing facilities use different size fractions of hot aggregate which are drawn in proportional amounts from storage bins to make up a single batch for mixing. The combination of aggregates is dumped into a mixing vessel. A proportional amount of asphalt is thoroughly mixed with the aggregate in the mixing vessel. After mixing, the material is then emptied into trucks, storage silos, or surge bins. The drum-mixing process heats and blends the aggregate with asphalt all at the same time in the drum mixer. When the mixing is complete, the hot-mix is then transported to the paving site and spread in a partially compacted layer to a uniform, even surface with a paving machine. While still hot, the paving mixture is further compacted by heavy rolling machines to produce a smooth pavement surface.
The quality of asphalt is affected by the inherent properties of the petroleum crude oil from which it was produced. Different oil fields and geographic regions produce crude oils with very different characteristics. The refining method also impacts the asphalt quality. For engineering and construction purposes, important factors include: consistency, also referred to as the viscosity or the degree of fluidity of asphalt at a particular temperature; purity; and worker safety.
The consistency or viscosity of asphalt varies with temperature, and asphalt is graded based on ranges of consistency at a standard temperature. Careless temperature and mixing control can cause more hardening damage to asphalt than many years of service on a roadway. A standardized viscosity or penetration test is commonly specified to measure paving asphalt consistency. Air-blown asphalts typically use a softening point test.
Purity of asphalt can be easily tested since it is composed almost entirely of bitumen, which is soluble in carbon disulfide. Refined asphalts are usually more than 99.5% soluble in carbon disulfide and any impurities that remain are inert. Because of the hazardous flammable nature of carbon disulfide, trichloroethylene (TCE), which is also an excellent solvent for asphalt, is used in the solubility purity tests.
Asphalt must be free of water or moisture as it leaves the refinery. However, transports loading the asphalt may have moisture present in their tanks or beds. This can cause the asphalt to foam when it is heated above 100° C., which poses a safety hazard. Specifications usually require that asphalts resist foaming at temperatures up to 175° C. If heated to a sufficiently high temperature, asphalt will release fumes which can flash in the presence of a spark or open flame. The temperature at which this occurs, the flashpoint, is well above temperatures normally used in paving operations. Because of the possibility of asphalt foaming and to ensure an adequate margin of safety, the flashpoint of the asphalt is measured and controlled.
Another important engineering property of asphalt is its ductility, which is a measure of a material's ability to be pulled, drawn, or deformed. The presence or absence of ductility is usually more important than the actual degree of ductility because some asphalt compositions having a high degree of ductility are also more temperature sensitive. Ductility is measured by an “extension” test, whereby a standard asphalt briquette molded under prescribed conditions and dimensions is pulled at a specified temperature (normally 25° C.) until it breaks under tension. The elongation at which the asphalt sample breaks is a measure of the ductility of the sample.
Related U.S. application Ser. No. 11/584,771, incorporated by reference herein, describes an enhanced solvent deasphalting process where a hydrocarbon oil feedstock containing asphaltenes is introduced into a mixing vessel with a paraffinic solvent and a solid adsorbent material. The solid adsorbent material can include attapulgus clay, alumina, silica activated carbon and zeolite catalyst materials, and combinations of those adsorbent materials. The solid asphaltenes formed in the paraffinic solvent phase are mixed with the adsorbent material for a time sufficient to adsorb sulfur- and nitrogen-containing polynuclear aromatic molecules on the adsorbent material. The solid phase comprising asphaltenes and adsorbent is separated from the oil/solvent mixture. The oil/solvent mixture is passed to a separation vessel to separate the deasphalted oil and paraffinic solvent, and recovere the solvent for recycling to the mixing vessel. The asphalt/adsorbent material mixture is passed to a filtration vessel with an aromatic or polar solvent to desorb the adsorbed compounds and to recover the solid asphalt phase. The aromatic or polar solvent mixture is then passed to a fractionator to recover the solvent. This process provides deasphalted oil with greater than 50 weight % reduction in sulfur and greater than 75 weight % reduction in nitrogen, compared to conventional solvent deasphalting processes that reduce only about 20 weight % of sulfur and 37 weight % of nitrogen.
During the various processes in a refinery, certain materials, typically those used as catalytic and non-catalytic adsorbent materials, must be reconditioned and/or removed after they are considered “spent,” i.e., their adsorbent capacity and/or catalytic activity falls below a desired efficacy. Adsorbent materials such as attapulgus clay, alumina, silica, activated carbon, silica alumina or zeolite catalyst material are used in the enhanced solvent deasphalting process described in U.S. application Ser. No. 11/584,771, and other intermediate refining processes including but not limited to hydrotreating, hydrocracking, and fluid catalytic cracking. The adsorbent includes constituents such as heavy polynuclear aromatic molecules, sulfur, nitrogen and/or metals. Disposal of these adsorbent as waste materials incurs substantial expense and entails environmental considerations.
In addition, when adsorbent materials are reconditioned, for example, by solvent desorption, heat desorption or pyrolysis at high temperatures, the process reject removed from the adsorbent materials must also be disposed of. These adsorbent materials can include heavy hydrocarbon molecules containing sulfur, nitrogen and/or heavy aromatic molecules, and metals such as nickel and vanadium.
Therefore, a need exists for a cost-effective solution for eliminating refinery process waste, including spent catalytic and non-catalytic adsorbent materials, as well as adsorbate process reject materials derived from desorption, while minimizing conventional waste handling demands.